Electro-optic high voltage sensor

ABSTRACT

A small sized electro-optic voltage sensor capable of accurate measurement of high levels of voltages without contact with a conductor or voltage source is provided. When placed in the presence of an electric field, the sensor receives an input beam of electromagnetic radiation into the sensor. A polarization beam displacer serves as a filter to separate the input beam into two beams with orthogonal linear polarizations. The beam displacer is oriented in such a way as to rotate the linearly polarized beams such that they enter a Pockels crystal having at a preferred angle of 45 degrees. The beam displacer is therefore capable of causing a linearly polarized beam to impinge a crystal at a desired angle independent of temperature. The Pockels electro-optic effect induces a differential phase shift on the major and minor axes of the input beam as it travels through the Pockels crystal, which causes the input beam to be elliptically polarized. A reflecting prism redirects the beam back through the crystal and the beam displacer. On the return path, the polarization beam displacer separates the elliptically polarized beam into two output beams of orthogonal linear polarization representing the major and minor axes. The system may include a detector for converting the output beams into electrical signals, and a signal processor for determining the voltage based on an analysis of the output beams. The output beams are amplitude modulated by the frequency of the electric field and the amplitude of the output beams is proportional to the magnitude of the electric field, which is related to the voltage being measured.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC07-94ID13223 between the United States Department ofEnergy and Lockheed Martin Idaho Technologies Company, now Contract No.DE-AC07-99ID13727 with Bechtel BWXT Idaho, LLC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems and devices forelectro-optically measuring and sensing voltages. More particularly, thepresent invention relates to systems and devices for sensing andmeasuring high voltages associated with electric fields produced byenergized conductors.

2. Relevant Technology

The ability to accurately sense and measure power is an important aspectof power systems and the power industry. Currently, however, powermeasurement and metering is typically performed only when necessary,which frequently occurs on the high voltage or power source side beforethe voltage is stepped down for distribution. As the power industryderegulates, it is becoming more important to accurately track andmeasure power, which indicates that additional measuring and metering isneeded in the power infrastructure. Power measurements are made bydetermining the values of both the current and the voltage. Whilecurrent measurements are easily performed and are readily available asmany current measurement devices are currently in place, voltagemeasurements are not readily available and can be rather difficult toaccurately obtain.

High voltage measurement is traditionally accomplished using iron coreferromagnetic potential transformers. Potential transformers, however,are problematic for a variety of reasons. They exhibit a limited dynamicrange, have limited bandwidth, and introduce a substantial degree ofnon-linearity. Also, potential transformers have been observed tounintentionally conduct dangerous levels of energy downstream towardsequipment or personnel thereby creating a serious safety hazard.

Many conventional methods for sensing and measuring high voltages,including potential transformers, require direct electrical contact withthe energized conductor, which has the major disadvantage of causinginterruptions or interference with the power transmission of a systemdue to the presence of an additional load. Prior voltage sensing andmeasuring systems also tend to be relatively bulky due to therequirement for a large voltage divider which is necessary to connectthe sensing element with the energized conductor. Large voltage dividersare not only space consuming, but are also expensive and difficult toimplement in many situations. For example, the installation of asubstation in a large city is quite difficult because the available realestate in which to install the substation is very limited. In otherwords, the ability to effectively measure high voltages as well as powercan be difficult and expensive.

Another favored method for measuring a voltage or a potential is relatedto the electric field associated with the potential. Open air electricfield sensors have been designed and built but are extremely susceptibleto factors such as changes in ambient dielectric constant, adjacentconductor voltages, and conducting objects such as traveling motorvehicles, which induce signals and noise which can interfere with oroverride the reading of the voltage to be measured.

Relatively recently, optical sensors have been designed for voltagesensing applications, such as those which utilize interferometricmodulation. Although relatively compact, such systems suffer fromextreme temperature sensitivity, which makes these types of systemsimpractical for many situations. Optical sensors which are mechanicallymodulated have also been attempted, but suffer from unreliability due tofailure of the moving parts in these systems.

Optical voltage sensors which operate by taking advantage of the Pockelseffect have also been developed. The Pockels effect is an electro-opticeffect which is manifested in certain crystalline materials which havethe property of advancing or retarding the phase of polarized lightwaves when a voltage or an electric field is applied to the crystallinematerial. The effect on the phase of the light wave is linearlyproportional to the first power of the applied voltage, which makes itideal for accurate voltage sensing and measuring applications.

Thus, the electromagnetic beam or light wave passing through the Pockelscrystal or cell undergoes an electro-optic effect when the crystal issubjected to an electric field. The electro-optic effect is observed asa differential phase shift, or a differential phase modulation, of theelectromagnetic beam components in orthogonal planes of theelectromagnetic radiation. The degree to which the electromagnetic beamis altered is indicative of the strength of the electric field. Bydetermining the amount of the alteration, or the amount of thedifferential phase, the voltage being measured can be determined.

In order to effectively measure a high voltage, a typical Pockels cellvoltage sensor requires a half-wave plate and a beam splitter. The beamsplitter is used to separate the orthogonal components of theelliptically polarized beam such that the differential phase can bedetermined. The half-wave plate is necessary to properly orient theelectromagnetic beam as it enters and leaves the Pockels crystal. If thehalf-wave plate is not positioned correctly, the voltage will not bemeasured accurately.

The half-wave plate is a both a critical part of current electro-opticvoltage sensors and a source of inaccurate voltage measurements. Theproblem with half-wave plates stems from their extreme sensitivity totemperature. For example, the half-wave plate is responsible forrotating the electromagnetic beam by before it enters the Pockels cell.The wavelength of the light wave passing through the half-wave plate isvery small, and the effect of temperature on the half-wave plate, whichalters the dimensions of the half-wave plate, can ultimately have asignificant effect on the accuracy of voltage measurements because thelight wave is not properly rotated to the desired degree.

Another problem associated with many electro-optic voltage sensors isthat they are inherently sensitive to temperature variations whichintroduce an intrinsic phase shift to the electromagnetic beam which ispassing through the crystal. The reason for the intrinsic phase shift isrelated to the bi-refringent properties of the Pockels crystals. Manyapplications utilize a crystal in which the indices of refraction arenot equal. As temperature changes, the indices of refraction alsochange. Unfortunately, the change in the indices of refraction is linearin each direction and the difference between the indices of refractionis thereby changing as the temperature changes. This change is bothdifficult to measure and ultimately remove from voltage calculations. Asa result, an accurate measurement requires that the temperature of thePockels crystal be constantly monitored. Otherwise, it is very difficultto account for the inherent differential phase shift which is therebyintroduced. Additionally, monitoring the temperature introduces unwantedcost.

It would therefore be a significant advantage in the art to provide acompact voltage sensing or measuring apparatus which: does not requiredirect electrical contact with the energized conductor; is capable ofaccurate operation under a wide range of variable temperatures andenvironmental conditions; is reliable; is cost effective; and issubstantially unaffected by temperature.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore an object of one embodiment of the present invention toprovide a device for the measurement of a voltage which does not requiredirect electrical contact with a conductor.

It is a further object of one embodiment of the present invention toprovide a voltage sensor device which is relatively insensitive totemperature and is capable of use in a wide variety of environmentalconditions.

It is yet another object of one embodiment of the present invention toprovide a voltage sensor apparatus which can accurately measure highlevels of voltage without a voltage divider.

It is a further object of one embodiment of the present invention toprovide a voltage sensor apparatus which is of relatively small size.

It is yet another object of one embodiment of the present invention toprovide a voltage sensor which is capable of being integrated withexisting power transmission and distribution equipment.

It is still a further object of one embodiment of the present inventionto provide a voltage sensor which uses the electro-optic Pockels effectwithout requiring a wave plate or an external beam splitter.

While the present invention is described in the context of a highvoltage sensor, it is to be understood that the present apparatus may beused in any type of electrical or optical application. The above objectsare realized in a specific illustrative embodiment of an electro-opticvoltage sensor device whereby the voltage difference (electricalpotential difference) between objects and positions may be measured.Voltage is a function of the electric field and the geometries,compositions, and distances of the conductive or insulating matter andwhen the effects of an electric field can be observed, a voltage can becalculated.

The sensor device can be utilized to sense and measure an electric fieldusing a beam of electromagnetic radiation which passes through thesensor. In order the effectively measure an electric field using anelectro-optic device, it is necessary to linearly polarize anelectromagnetic radiation or input beam. This is accomplished using amaterial such as calcite that exhibits bi-refringent properties. Thus,an input beam is physically split into two separate and orthogonallinearly polarized beams whose polarizations are parallel to, forexample, the x and y axes of the calcite.

The calcite is connected to a crystal which exhibits the Pockels effectin the presence of an electric field. The calcite is oriented such thatone of the linearly polarized beams is effectively rotated byapproximately 45 degrees as it impinges the Pockels cell. In otherwords, the x axis of the calcite is offset 45 degrees from the x axis ofthe Pockels cell. As the beam travels through the Pockels crystal, theelectric field causes the beam to become elliptically polarized. Thebeam is reflected back through the Pockels crystal towards the calciteusing a prism or other reflecting device. Reflecting the beam throughthe Pockels cell a second time has the advantages of adding to thePockels effect and increasing the sensitivity of the sensor.

As the beam exits the Pockels crystal and enters the calcite, it isphysically separated into two output beams which correspond to the majorand minor axes of the elliptically polarized beam. As stated, thecalcite separates these components because of the bi-refringent propertyof the calcite. The output beams are collected and analyzed to determinethe peak-to-peak value of the voltage as well as the root-mean-square(RMS) value of the voltage. The output beams are 180 degrees out ofphase, but have equal magnitudes and are amplitude modulated by thefrequency of the electric field.

The input and output beams are collimated using, for example, a gradedindex lens attached either to the sensor or to the fiber optic cableswhich carry the input and output beams to and from a detector. Thedetector both supplies the input beam and receives the output beams.

An important aspect of the orientation of the calcite portion of thesensor is that it eliminates the need for both a waveplate and a beamsplitter. The orientation of the calcite block with respect to thePockels crystal effectively provides the rotation of the linearlypolarized input beam which was previously provided by the waveplate.However, the effective rotation provided by the calcite is nottemperature dependent. Thus, the beam rotation, which was previouslyattributable to the variations of the wave plate caused by temperatureis avoided. Further, the Pockels cell is preferably a z-cut crystalhaving indices of refraction in the x and y directions that are equalwhen the crystal is static. This eliminates the phase differentialintroduced by crystals having non-equal indices of refraction whichchange with respect to temperature.

The output beams are received by photo detectors which convert theoptical signals into electrical signals. The two or more amplitudemodulated signals can be processed in an analog or digital circuit, orboth. The amplitude modulated signals may be, for example, converted todigital signals, fed into a digital signal processor (DSP), andprocessed into a signal proportional to the voltage which produced theelectric field. The amplitude modulated signals could also be opticallyprocessed. Alternatively, the output of the analog circuit, which may bein the form of a sinusoidal signal, may be used to calculate thepeak-to-peak voltage and an RMS voltage values of the voltage beingmeasured.

Advantages of the present invention will become apparent from thefollowing description, taken in connection with the accompanyingdrawings, wherein, by way of illustration and example, embodiments ofthe present invention are disclosed.

The drawings constitute a part of this specification and includeexemplary embodiments to the invention, which may be embodied in variousforms. It is to be understood that in some instances various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an environment in which a preferredembodiment of an electro-optic voltage sensor system may be implemented;

FIG. 2a is a block diagram illustrating both the orientation of a beamdisplacer with respect to a Pockels crystal and the respective axes ofthe beam displacer and the Pockels crystal;

FIG. 2b is a block diagram illustrating a linearly polarized beam thatis a planar wave propagating through the beam displacer;

FIG. 2c illustrates how the linearly polarized beam impinges the Pockelscrystal at a 45 degree angle;

FIG. 3a is an end view of an electro-optic voltage sensor device andillustrates the orientation of the input and output structures as wellas the orientation of a beam displacer with respect to a Pockels crystaland a prism;

FIG. 3b is a source side view of a preferred embodiment of anelectro-optic voltage sensor device and illustrates the path of an inputbeam and a displaced beam;

FIG. 3c is a return side view of a preferred embodiment of anelectro-optic voltage sensor showing the path of a return beam ofelectromagnetic radiation after being reflected by a prism;

FIG. 3d is a top side view of a preferred embodiment of theelectro-optic voltage sensor showing the input beam and the output beamsas they propagate through the sensor device; and

FIG. 3e is an end view of a voltage sensor associated with FIG. 3d whichillustrates the orientation of the input and output structures as wellas the position of the input and output beams propagating through thevoltage sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Voltages, potentials and electric fields are present in a variety ofdifferent systems. They can be found in airplanes, locomotives, radarmapping applications, electric field mapping applications, lightningdetection applications, and in applications for sensing highfrequencies. In particular, voltages and electric fields are present inpower systems and are a major concern of the power industry. Becausevoltages, electric fields and the associated power are an integral partof many applications, it is vital that they be accurately measured orsensed.

As described previously, sensing and measuring voltages or electricfields has proven problematic for a variety of reasons. For example, asthe power needs of large cities grows, it is necessary to either expandor create new substations such that the power may be safely distributed.Part of the expansion or creation of a substation involves theinstallation of both power and potential transformers. The potentialtransformers are used to measure the voltage of the power transformers.A safe installation is subject to a myriad of requirements includingsufficient real estate to accommodate the bulky transformers. Sufficientreal estate is a scarce commodity in large cities, which makes it bothdifficult and expensive to expand or install power substations. Anotherproblem is that many existing devices utilized to sense and measurevoltages require a direct electrical connection to the conductor havingthe voltage or potential.

The present invention is directed to systems and apparatus for sensingand measuring high voltages without requiring direct electrical contactwith a conductor. In particular, the present invention utilizes thePockels effect to sense and measure an electric field, from which acorresponding voltage can be calculated. The present invention does notrequire a direct electrical connection and provides a safe and compactmeasuring system for high levels of voltage. The electro-optic voltagesensor of the present invention is capable of providing accuratemeasurements under a wide variety of environmental conditions, and isparticularly suitable for environments generating high voltages, such asthose present in power substations.

As used herein “input beam” is intended to refer to electromagneticradiation, including electromagnetic radiation having wavelengths withinthe visible spectrum or beyond the visible spectrum. Input beam furtherrefers to monochromatic light and is intended to encompass all beams andsignals that have electromagnetic properties. As used herein “outputbeam” is intended to refer to an input beam that has been subjected toan electric field. The effect of the electric field on an input beam isevident in the output beam, which enables the magnitude of the electricfield and associated voltage to be accurately measured.

FIG. 1 illustrates an environment in which an apparatus or system forsensing and measuring a voltage may be implemented. Electro-opticvoltage sensor system 30 is illustrated as being capable of sensing thepotential or voltage present on voltage source 20. Voltage source 20 hasan energized conductor 22 and a grounded conductor 26, which isfrequently covered with sheath 28. In addition, voltage source 20 mayhave an insulator 24, which may have dielectric properties. Voltagesource 20 is illustrated as a high voltage cable and is intended to bean example of a device which has a high voltage present and is a sourceof an electric field. Other potential devices include a shielded cablejoint, a through-hole insulator, a shielded bus, an insulatedswitchgear, a duct enclosed bus, or any other device or apparatus whichgenerates an electric field. Voltage source 20 includes apparatuspresent at power substations from which a high voltage measurement maybe taken.

Conductor 22 typically has a higher potential than conductor 26 and as aresult, a measurable potential or voltage is present between conductor22 and conductor 26. Accordingly an electric field exists betweenconductor 22 and conductor 26 that is dependent on the geometries,compositions, and distances of the conducting and insulating materials.As illustrated, the electric field begins on conductor 22 and extendsradially to conductor 26, where the electric field is terminated.

The ability of system 30 to sense and measure a voltage and an electricfield begins with sensor 32. Sensor 32 receives, via fiber optic cable38, an input beam, such as laser light, monochromatic light, or otherbeam, which is generated by light source 40. The input beam istransmitted from light source 40 into sensor 32, which is positioned inthe electric field generated by voltage source 20. As the input beamtravels through sensor 32, it is acted upon by the electric field, whichcreates an output beam having a differential phase between thecomponents of the beam propagating along orthogonal axes. The electricfield is capable of modulating the output beam. The output beam thenexits sensor 32 via fiber optic cables 34 and 36 and is collected byphoto detectors 42 and 44. Fiber optic cables 38, 34, and 36 are capableof optically communicating an input or output beam to and from sensor32. Fiber optic cable 38 is preferably a single mode optical fiber andfiber optic cables 34 and 36 are preferably multi-mode optical fibers.

After the output beams are transmitted to photo detectors 42 and 44,they are processed by signal processor 48. Signal processor 48 maycomprise a computer or other device capable of processing the electricalsignals provided by photo detectors 42 and 44. The information fromsignal processor 48 may be illustrated on instrumentation 50. Signalprocessor 48 is able to compute the voltage present across conductor 22and conductor 26 based on the information provided by the output beams.Signal processor 48 is also capable of controlling detector 49, whichcomprises signal processor 48, drive electronics 36, light source 40 andphoto detectors 42 and 44.

As further illustrated in FIG. 1, sensor 32 is preferably placed betweenconductor 22 and conductor 26 and sensor 32 is further illustrated asbeing placed near ground conductor 26. Placing sensor 32 in this mannereliminates the effect of external electric fields on sensor 32 becausethey are shielded from sensor 32 by conductor 26. Further, sensor 32 maybe placed in physical contact with conductor 26 or 22, but it is not indirect electrical contact with either conductor 22 or conductor 26.Alternatively, sensor 32 need not be in physical contact with eitherconductor 22 or conductor 26. In other words, sensor 32 is isolated fromthe conductors and does not present a load to voltage source 20.

Sensor 32 is optically isolated from conductor 22 and will not conductpower away from voltage source 20. The effect of the electric field onan input beam passing through sensor 32 is evident in the differentialphase of the input beam. Because the input beam has a differentialshift, it is amplitude modulated by the frequency of the electric field.The amplitude of the resultant output beam is proportional to themagnitude of the electric field, which in turn is proportional to themagnitude of the voltage being measured. As a result, the peak-to-peakvalue and root-mean-square (RMS) values of the voltage are easilydetermined.

FIG. 2a is a schematic diagram illustrating the orientation of beamdisplacer 60 with respect to crystal 70. The axes of beam displacer 60are illustrated as x axis 66, y axis 62 and z axis 64. The z axis 64 isthe direction of propagation for both input and output beams. Beamdisplacer 60, in a preferred embodiment, is calcite, which is abirefringent material. The bi-refringence of beam displacer 60 causes aninput beam to be separated into orthogonal linearly polarized beams.

The axes of crystal 70 are illustrated as x axis 74, y axis 72 and zaxis 76, where the z axis 76 is illustrated as the direction ofpropagation for both input and output beams. This orientation isreferred to as a z-cut crystal, but other crystal cuts may be used suchthat the direction of propagation is any of the available axes. In otherembodiments, the direction of propagation may not coincide exactly withan axis. Crystal 70, in a preferred embodiment, is a magnesium oxide(MgO) doped lithium niobate (LiNbO₃) crystal. In the absence of anelectric field, the index of refraction for both x axis 74 and y axis 72are equal. In the presence of an electric field, the index of refractionparallel to the electric field is changed such that velocity of acomponent of the input beam that is parallel to the electric field isretarded. As a result, a linearly polarized input beam which impingescrystal 70 at a 45 degree angle exhibits a phase differential that isrelated to the magnitude of the electric field.

FIG. 2b illustrates a portion of an input beam and is shown as component102. Component 102 is linearly polarized and is parallel to an axis ofbeam displacer 60. The other component of an input beam which is alsolinearly polarized is illustrated in FIG. 3a. FIG. 2c illustratescomponent 102 as it exits beam displacer 60 and impinges crystal 70. Theorientation of beam displacer 60 with respect to crystal 70, asillustrated in FIG. 2a, causes component 102 to impinge crystal 70 atangle 78, which is preferably 45 degrees from x axis 74.

Mathematically, component 102 can be represented as having majorcomponent 71 and minor component 73, which are parallel with x axis 74and y axis 72 respectively. In the presence of an electric field, thepropagation of either major component 71 or minor component 73 isdelayed, which introduces a differential phase shift into component 102.Thus, the wavefront of component 102 may be viewed as ellipticallypolarized. The differential phase shift, in combination with thefrequency of the electric field results in an amplitude modulated outputbeam, whose amplitude is proportional to the magnitude of the electricfield. In this instance, the high frequency of component 102 is thecarrier frequency whose amplitude varies according to the modulatingfrequency, which is the frequency of the electric field in thisinstance. In power applications, the modulating frequency is frequently60 Hz.

FIG. 3a is an end view of sensor 32 and illustrates the preferableconfiguration of the input and output beams. FIG. 3a also illustrates anend view of the beams as they are oriented in sensor 32. As shown, theend view illustrates beam displacer 60, crystal 70 and prism 80. Thephysical arrangement illustrated in FIG. 3a is important in order toeffectively measure a voltage. As described previously, beam displacer60 is oriented 45 degrees with respect to crystal 70. As an input beamenters beam displacer 60, it is split into two linearly polarized beams:input beam 90 and displaced beam 91. The physical orientation of thecomponents of sensor 32 allow displaced beam 91 to be discarded withoutentering crystal 70 or prism 80.

FIG. 3b is a source side view of sensor 32. The body of sensor 32comprises a beam displacer 60, crystal 70 and prism 80. Beam displacer60 has an input 61, which is connected to fiber optic cable 38. Theoperation of sensor 32 begins when an input beam is presented by fiberoptic cable 38 to sensor 32 through input 61 Fiber optic cable 38 has agraded index (GRIN) lens attached to it which serves to collimate theinput beam as the input beam enters beam displacer 60 and the GRIN lensis an example of collimating means for collimating electromagneticradiation. As previously mentioned, a significant characteristic of beamdisplacer 60 is that it is bi-refringent. This characteristic indicatesthat beam displacer 60 has a different index of refraction alongdifferent axes. As a result, the input beam is divided into two separatelinearly polarized beams indicated by beams 90 and 91.

The input beam generated by light source 40 in FIG. 1 and carried byfiber optic cable 38 to beam displacer 60 is preferably circularlypolarized, but may have any polarization. Thus, the input beam isseparated by beam displacer 60 to provide two physically separatedplanes of polarization. Beam 90 and beam 91 are both linearly polarizedand are orthogonal with respect to each other. In this example, beam 91is discarded and exits beam displacer 60 without entering crystal 70.Beam 91, which is a displaced beam, therefore does not enter crystal 70or prism 80 and does not introduce any potential noise into themeasurement of the voltage. Beam displacer 60 therefore serves as afilter which produces a linearly polarized input beam and is an exampleof filtering means for filtering a circularly polarized input beam toproduce a linearly polarized input beam. Effectively, beam displacer 60,or the filtering means limits the input beam that enters crystal 70 to asingle planar wave.

The orientation of beam displacer 60 with respect to crystal 70 is alsosignificant as previously described. Beam displacer 60 eliminates theneed of a half-wave waveplate because the linearly polarized input beam90 enters or impinges crystal 70 at an angle substantially equal to 45degrees because of the orientation of beam displacer 60 with respect tocrystal 70. In other words, beam 90 is a linearly polarized input beamwhich is parallel to an axis of beam displacer 60, and as beam 90 exitsbeam displacer 60, it enters crystal 70 at an angle of approximately 45degrees with respect to the x and y axes of crystal 70. Thus, thefunction of a waveplate, which is to rotate the input beam by anappropriate amount, is effectively performed by beam displacer 60. Thewaveplate is extremely temperature dependent, and the elimination of thewaveplate from sensor 32 removes the effect of temperature on the beamrotation of beam 90 which was previously attributable to the half-wavewaveplate. The overall effect is to improve the accuracy of the voltagemeasurement.

After beam 90 enters crystal 70, it is subject to the effects ofelectric field 18 and is denoted as beam 92. Beam 92, which can berepresented as having a first component and a second component of equalmagnitudes which are respectively parallel to the x and y axes ofcrystal 70. The electric field alters the index of refraction of oneaxis, which has the effect of slowing or retarding that component ofbeam 92. As a result, a phase differential is introduced into beam 92and beam 92 becomes elliptically polarized.

The phase differential allows beam 92 to be amplitude modulated byelectric field 18. The modulating frequency is the frequency of electricfield 18 and the carrier frequency is equal to the frequency of beam 92.The amplitude modulation is linearly related to the magnitude of theelectric field and a measurement of the amplitude enables the voltage tobe calculated.

FIG. 3c is a return signal side view of sensor 32 and illustrates beam92 as it travels back through crystal 70 and beam displacer 60, afterbeing reflected by prism 80. The return path adds to the Pockels effectand increases the sensitivity of sensor 32. As beam 92 exits crystal 70and re-enters beam displacer 60, the elliptically polarized beam 92 isseparated into two linearly polarized output beams 93 and 94, which areout of phase by 180 degrees, but have equal magnitudes. Output beams 93and 94 are representative of the major and minor axes of beam 92. Asdescribed previously, the separation of the components of beam 92 ispossible due to the bi-refringent property of beam displacer 60. In thisinstance, beam displacer 60 is an example of displacement means fordirecting an output beam. Other examples of displacement means include abeam splitter or other materials which separate the components of anoutput beam including calcite. Displacement means is also intended tocover materials having the property of separating an input or outputbeam into linearly polarized components, which may or may not beorthogonally related.

One of the output beams 93 and 94 is received at output 63 and the otheroutput beam is received at output 65. Outputs 63 and 65 are alsoequipped with GRIN lenses to collect the beams into the fiber opticcables 34 and 36 as the output beams exit beam displacer 60. Thus, beamdisplacer 60 also serves as a beam splitter and is an example of beamsplitting means for separating orthogonal components of an ellipticallyor other wise polarized beam.

Crystal 70 is preferably a z-cut MgO doped lithium niobate crystal. Theadvantage of the z-cut lithium niobate crystal is that the index ofrefraction is equal for the x and y axes when the crystal is not subjectto an electric field. This is important because in a crystal where theindices of refraction that are orthogonal to the direction of wavepropagation are not equal, temperature differences can introduce anintrinsic phase differential that is related to temperature rather thanstrength of the electric field. In the example illustrated in FIG. 3a,the indices of refraction in the x and y axes are equal, whicheliminates the effect of temperature on the intrinsic phasedifferential. In sum, the effect of temperature, with respect to sensor32, is essentially eliminated by the present invention.

FIG. 3d is a side view of sensor 32 and more fully details the path ofan input beam, the displaced beam and the output beams. An input beam istransmitted by fiber optic cable 38 to input 61, which is a GRIN lens inone embodiment that effectively collimates the input beam such that theinput beam is not dispersed but is directed through sensor 32. Asillustrated, the input beam is separated into beam 90 and beam 91 bybeam displacer 60. Beam 90 and beam 91 are linearly polarized beams thatare propagating along orthogonal planes. As illustrated, beam 91 isdiscarded and does not enter crystal 70. As described above, theorientation of beam displacer 60 with respect to crystal 70 causes beam90 to enter crystal 70 at a rotated angle with respect to the axes ofcrystal 70. The preferred angle of rotation is 45 degrees such that beam92 may be represented by vectors in the x and y directions which areequal in magnitude but different in phase as beam 92 experiences theelectric field.

Beam 92 then encounters prism 80, which functions to reflect beam 92back through crystal 70 as illustrated in FIG. 3d, which illustrates thereturn path of beam 92 to outputs 63 and 65. Prism 80 is an example ofreflecting means for reflecting beam 90. Other examples of reflectingmeans include a mirror or an optical fiber. As beam 92 returns throughcrystal 70, electric field 18 continues to impose a differential phaseshift on the minor and major axes of elliptically polarized beam 92.

In the elliptically polarized beam 92 which is propagating throughcrystal 70, beam 92 has at least two components which propagate along atleast two orthogonal planes respectively. The phase of the components ineach plane of propagation are the object of a shift relative to thephase of the component in the other plane. The Pockels electro-opticeffect changes the relative phases of the components of beam 92 byaltering their respective velocities along the different axes of crystal70. The magnitude of the differential phase shift is proportional to themagnitude of electric field 18, which is related to the voltage beingmeasured.

In practice, sensor 32 is encased in a dielectric material whichimproves uniformity in the electric field, particularly around the edgesof crystal 70. This encourages the uniformity in the phase shift of thebeam passing through crystal 70 and minimizes the stress of the voltageon the materials in and surrounding sensor 32.

As further illustrated in FIG. 3d, the return path of beam 92 causesbeam 92 to re-enter beam displacer 60. Again, the orientation of beamdisplacer 60 with respect to crystal 70 and the bi-refringent propertyof beam displacer 60 causes the major and minor axes of beam 90 toseparate and travel as linearly polarized beams 93 and 94. Beams 93 and94 are orthogonal with respect to each other. Thus, beam displacer 60again performs the function of a half-wave plate without the effects oftemperature. Beams 93 and 94 are received at outputs 63 and 65respectively. Outputs 63 and 65 typically collect beams 93 and 94 usinga GRIN lens and focus the collected beams on a fiber optic cable. Asillustrated in FIGS. 3a and 3 e, input 61 and outputs 63 and 65 areconveniently located at the same end of sensor 32 and outputs 63 and 65and input 61 are examples of structural elements through which an outputbeam or an input beam may be communicated to or from sensor 32. The useof beam displacer 60 eliminates the need for a beam splitter to separatethe orthogonal components of beam 90, which allows for a more compactelectro-optic voltage sensor.

In FIG. 3d, the output signals are represented as output signal 200 andoutput signal 202. Each output is an amplitude modulated signal that isrepresentative of either the major or minor axis of the ellipticallypolarized light produced from the effects of the electric field. Outputsignals 200 and 202 are collected using a GRIN lens or other collectingdevice and transmitted to detector 49 illustrated in FIG. 1. Theamplitude modulated output signals are received by photo detectors 42and 44 where they are converted to electrical signals and are referredto herein as component A and component B. Because FIG. 3d is a top sideview, FIG. 3e illustrates an end view of the orientation of the inputand output beams associated with the top view of FIG. 3d.

FIG. 3e again illustrates the positioning of beam displacer 60, crystal70 and prism 80 such that the propagating beam may be used to measurethe voltage associated with the electric field. Also, in FIGS. 3b, 3 cand 3 d, electric field 18 is illustrated. Electric field 18 is notintended to indicate the direction of the electric field. Rather,electric field 18 is intended to illustrate the presence of an electricfield.

It is important to understand that the voltage being measured can bedetermined using only one of the output beams. In fact, the output beamsare equal in magnitude, but out of phase by 180 degrees. Using componentA and component B to compute the voltage allow power fluctuations in thepower of the light source to be eliminated from the calculation of thevoltage. The voltage being measured is determined from a signal equal tothe difference of components A and B divided by the sum of components Aand B. The signal is a sine squared signal and the peak-to-peak outputcan be determined using an arcsine function. The RMS value can bedetermined from the peak-to-peak output.

One problem associated with this computation is that the power of thelight source may vary, or may be affected by the physical components inuse. This can be remedied using a scaling factor in software whichaccounts for offset and gain components. The scaling factor is easilydetermined because the components A and B are presumably equal. Thescaling factor is used to make components A and B equal before anycalculations are performed. An offset scaling factor may also beintroduced if the input beam does not enter the Pockels cell or crystalat exactly 45 degrees. Another factor is often used to compensate forthe variations in the electric field which are attributable totemperature changes in the dielectric which may exist in the electricfield (illustrated as insulator 24 in FIG. 1).

The present invention may also be utilized in power computations incombination with a current sensor. The present invention as describedherein has advantages which include: small size and weight, increasedsafety from high voltage hazards, linearity, large dynamic range, highbandwidth, no direct contact required for sensing and no voltage dividerhardware requirements, compatibility with past, present and future powersubstation control and metering systems, temperature insensitivity, andrelatively low cost.

It is to be understood that the above described arrangements areintended as illustrative embodiments of the present invention and arenot intended to limit the invention. Numerous modifications andalternative arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the present invention.Thus, while the present invention has been shown in the drawings anddescribed hereinabove with particularity and detail in connection withwhat is presently deemed to be the most practical and preferredembodiment of the invention, it is apparent that numerous modificationsincluding, but not limited to, size, material, shape, form, function,and manner of operation may be made without departing from theprinciples and concepts set forth herein.

We claim:
 1. A system, in an environment having a voltage and an electric field, for measuring the voltage, the system comprising: (a) a detector comprising: (i) a light source for generating an input beam; and (ii) a first and second photo detector for receiving a first and second output beam; and (b) a sensor operably connected to the detector, the sensor comprising: (i) a crystal for receiving a first component of the input beam, wherein the crystal is oriented in the electric field such that the first component experiences a phase differential to produce an elliptically polarized output beam; and (ii) a beam displacer connected to the crystal and oriented at an angle with respect to the crystal, wherein the beam displacer: (A) receives the input beam from the light source and separates the input beam into the first and second components, wherein the first and second components are orthogonal and the first component impinges the crystal at the angle; and (B) receives the elliptically polarized output beam from the crystal and separates the elliptically polarized output beam into the first and second output beams which represent a major axis and a minor axis of the elliptically polarized output beam; and (c) a signal processor for receiving at least one signal from the first and second photo detectors, wherein the signal processor computes the voltage from the at least one signal.
 2. A system as defined in claim 1, wherein the input beam is circularly polarized electromagnetic radiation.
 3. A system as defined in claim 1, wherein the first and second output beams are linearly polarized electromagnetic radiation.
 4. A system as defined in claim 1, wherein the crystal is z-cut MgO doped lithium niobate and the x and y indices of refraction are equal when the crystal is not subject to the electric field.
 5. A system as defined in claim 1, wherein the sensor further comprises a prism connected to the crystal, wherein the elliptically polarized output beam is reflected back into the crystal towards the beam displacer.
 6. A system as recited in claim 5, wherein the crystal separates the prism from the beam displacer.
 7. A system as defined in claim 1, wherein the beam displacer causes the first component to impinge the crystal at the angle, wherein the angle is 45 degrees.
 8. A system as recited in claim 7, wherein an orientation of the beam displacer with the crystal causes the first component to impinge the crystal at said angle.
 9. A system as recited in claim 8, wherein the beam displacer and the crystal are coaxially aligned.
 10. A system as defined in claim 1, wherein the sensor further comprises: an input graded index lens operably connected to the light source for collimating the input beam; and a first and second output graded index lens operably connected to the at least one photo detectors for collecting the first and second output beams.
 11. A system as defined in claim 1, wherein the beam displacer comprises calcite, and wherein the x and y indices of refraction of the calcite are not equal.
 12. A system as recited in claim 1, wherein the beam displacer comprises a birefringent material.
 13. A system, in an environment having a voltage and an electric field, for measuring the voltage, the system comprising: (a) a detector comprising: (i) a light source for generating an input beam; and (ii) a first and second photo detector for receiving a first and second output beam; and (b) a sensor operably connected to the detector, the sensor comprising: (i) a crystal for receiving a first component of the input beam, wherein the crystal is oriented in the electric field such that the first component experiences a phase differential to produce an elliptically polarized output beam; and (ii) a beam displacer that is connected to the crystal at an offset angle of orientation with respect to the crystal, the beam displacer comprising a birefringent material, and wherein the beam displacer: (A) receives the input beam from the light source and separates the input beam into the first and second components, wherein the first and second components are orthogonal and the first component impinges the crystal at the angle; and (B) receives the elliptically polarized output beam from the crystal and separates the elliptically polarized output beam into the first and second output beams which represent a major axis and a minor axis of the elliptically polarized output beam; and (C) a signal processor for receiving at least one signal from the first and second photo detectors, wherein the signal processor computes the voltage from the at least one signal (c) a prism that is connected to the crystal and that is separated from the beam displacer by the crystal, wherein the elliptically polarized output beam is reflected back into the crystal towards the beam displacer.
 14. A system as defined in claim 13, wherein the input beam comprises circularly polarized electromagnetic radiation.
 15. A system as defined in claim 13, wherein the first and second output beams comprise linearly polarized electromagnetic radiation.
 16. A system as defined in claim 13, wherein the crystal comprises z-cut MgO doped lithium niobate and the x and y indices of refraction are equal when the crystal is not subject to the electric field.
 17. A system as defined in claim 13, wherein the beam displacer and the crystal are coaxially aligned, and wherein the offset angle of orientation comprises a rotational angle of approximately 45 degrees.
 18. A system as defined in claim 13, wherein the sensor further comprises: an input graded index lens operably connected to the light source for collimating the input beam; and a first and second output graded index lens operably connected to the at least one photo detectors for collecting the first and second output beams.
 19. A system as defined in claim 13, wherein the beam displacer comprises calcite, and wherein the x and y indices of refraction of the calcite are not equal. 